1. Field of Invention
The invention relates to a fabrication method for a thin film bulk acoustic-wave resonator (FBAR) and more particularly, the filters and duplexers made by such resonators.
2. Related Art
Filters are widely used in wireless communications. The demand is roughly estimated to be tens of billions annually. The commonly used filters include LC filters, dielectric filters, surface acoustic wave (SAW) filters, and thin film bulk acoustic-wave (BAW) filters. Since the thin film bulk acoustic-wave filter is featured in its small size, high-frequency capability, low insertion loss, and being able to readily integrate with integrated circuits (ICs), it is expected to have a wider applications in the future. The thin film bulk acoustic-wave filter is composed of thin film bulk acoustic wave resonator (FBAR). A typical FBAR is mainly composed of top/bottom electrodes and piezoelectric layers (e.g., PZT, AlN and ZnO) in between. Sometimes, a supporting or temperature compensation layer is also used. Piezoelectric layers are used here to make transduction between the electrical signal and acoustic wave. In order to effectively trap the energy inside the resonator, some special structure is made to reflect the elastic wave and trap them by means of the impedance difference. The special structure is usually an air cavity underneath the bottom electrode made by micro electro-mechanic system (MEMS) techniques. Another way to make low impedance reflection structure is to create a Bragg reflector. The latter method requires high film quality of each layer.
Currently, the cavity of the thin film bulk acoustic wave resonator is usually made using MEMS technology. There are two ways to implement it: bulk micromachining and surface micromachining. Please refer to FIG. 1 for the surface micromachining. This method makes a bridge 11 above the substrate 10 and removes the sacrificial layer 12 below the bridge 11 later to form a reflection cavity of the resonator. However, this method is not widely adopted because removing the sacrificial layer 12 from underneath the bridge 11 is a difficult job and the bridge structure can be easily damaged, too.
Most popular fabrication method is the bulk micromachining. One can etch from the backside of a substrate, leaving a very thin structure layer. Alternatively, one may directly dig a cavity underneath FBAR structure from the front side of substrate. In this case, the process integration is an important issue. Several drawbacks, however, exist for these conventional fabrication methods. The etching solutions usually used, for example, may have compatibility problems with existing CMOS processes and cause contamination problems. The anisotropic characteristics of some etching solutions are widely used to make precise shape control but will limit the shape choices for the cavity. Nevertheless, the etching solution may damage the already formed structure and the circuit.
FIG. 2 shows a schematic cross-sectional view of a FBAR fabricated using the conventional bulk micromachining. First, an FBAR structure 22 is formed on a substrate 20. Afterwards, the substrate 20 is etched from the backside until a very thin structure layer 21 is formed. This backside etching method has its advantage of easy fabrication. However, it requires a larger fabrication area and the wafer becomes brittle after etching, causing handling problems.
Please refer to FIGS. 3A to 3E for another excavation micromachining method developed by Agilent (disclosed in U.S. Pat. No. 6,060,818). This method first etches a substrate 30 from the front side by bulk micromachining to obtain a desired cavity 31 (FIG. 3A). The cavity 31 is then oxidized (by thermal oxidation, not shown in the drawing) and filled with a phosphosilicate glass (PSG) layer 32 (FIG. 3B). As shown in FIG. 3C, the PSG layer 32 is polished using the chemical-mechanical polishing (CMP) method. Thin film bulk acoustic-wave resonator structures 33 are then formed on the polished PSG layer 32 (FIG. 3D). Afterwards, a diluted H2O:HF solution is used to rapidly remove PSG layer 32 and create the cavity 34 required for forming the working resonators (FIG. 3E). This method etches from the front side and has the advantage of a small area. Such a method has some problems, such as a complicated fabrication procedure and limited cavity shapes if anisotropic etchants are used.
Porous silicon fabrication methods are also used to implement FBAR, as shown in FIG. 4. Porous silicon 41 is formed on the Si substrate first. Thin film bulk acoustic-wave resonator (FBARs) structures 42 are then formed on the porous silicon 41. Finally, the porous silicon 41 under the FBARs is removed using 2% NaOH. This method requires an n/n+/n structure. The etching solution, NaOH (or KOH), is not accepted by CMOS fabrication lines. Moreover, this method requires a complicated manufacturing process and a higher fabrication cost.